(i) Field of the Invention
This invention relates to method and apparatus for detecting vapours of one gas in another gas. More particularly, it is directed to method and means for detecting toxic or other noxious gases in air.
(ii) Description of the Prior Art
There is presently a need to detect vapours of chemical warfare agents (particularly vapours of the "so-called" G. and V-agents) in air down to very low concentrations. To be useful in the field, such detection should be made rapidly, i.e. taking no more than 10 minutes, and should be made using a simple apparatus which can be easily carried and used by untrained soldiers in the field.
There are two ways for detecting such vapours in air. The first involves the detection of the presence of dangerous concentrations of vapours of the agent. The second involves the detection of the absence of dangerous concentrations of vapours of the agent. In the first way, the test is used to provide a positive indication of concentrations producing militarily significant effects down to the threshold level, as well as to provide a readily recognizable negative indication of lower concentrations. In the second way, the test is used to provide a positive indication of concentrations lower than those producing militarily significant effects, as well as to provide a readily recognizable negative indication of higher concentrations.
As used in the present specification, the term "militarily significant effects" is intended to mean the developement, in a small fraction (1-5%) of exposed personnel, of no more than mild symptoms of poisoning by the agent. The appearance of such mild symptoms is related to the rate at which agent enters the body. This is a function of the breathing rate, the vapour concentration, and the body collection efficiency. The requirement is for detection of concentrations which do not exceed the above criteria of casuality production in resting or mildly active men exposed for an indefinite time. This indefinite time may be further defined as 12 hours.
Means for detection of such vapours in air which are currently available involve the provision of a bibulous material (e.g. absorbent paper) impregnated with a colorimetric agent, e.g. an enzyme which reacts with the particular agent being detected to provide a colour indication. The precise nature of such colorimetric agent is not important since it has no bearing on the present invention. However, in general, it can be said that in one such chemical system, contact between the impregnated bibulous material (e.g. paper) and air results either in the development of colour (absence of agent), or no development of colour (presence of agent) on the paper. In another such chemical system, contact between the impregnated bibulous material (e.g. paper) and air results in colour being developed very rapidly. If this colour persists for more than 2 minutes, agent is present, if the colour fades to white within the 2 minute period, agent is absent.
Contact between the air and the bibulous material may be achieved either by pulling an air sample therethrough with a handpump, or by waving the detector (i.e. the impregnated paper) in the air.
All the above means will detect low concentrations of the vapours of the agents but their sensitivities are not adequate. To detect the very much lower concentrations which do not produce symptoms after 12 hour exposure, the techniques of quantitative analysis must be employed. Such techniques are complicated and time-consuming. In addition, they must be performed by a trained analyst. Such approaches, therefore, are unsuited for field use.